Formation and control of fluidic species

ABSTRACT

This invention generally relates to systems and methods for the formation and/or control of fluidic species, and articles produced by such systems and methods. In some cases, the invention involves unique fluid channels, systems, controls, and/or restrictions, and combinations thereof. In certain embodiments, the invention allows fluidic streams (which can be continuous or discontinuous, i.e., droplets) to be formed and/or combined, at a variety of scales, including microfluidic scales. In one set of embodiments, a fluidic stream may be produced from a channel, where a cross-sectional dimension of the fluidic stream is smaller than that of the channel, for example, through the use of structural elements, other fluids, and/or applied external fields, etc. In some cases, a Taylor cone may be produced. In another set of embodiments, a fluidic stream may be manipulated in some fashion, for example, to create tubes (which may be hollow or solid), droplets, nested tubes or droplets, arrays of tubes or droplets, meshes of tubes, etc. In some cases, droplets produced using certain embodiments of the invention may be charged or substantially charged, which may allow their further manipulation, for instance, using applied external fields. Non-limiting examples of such manipulations include producing charged droplets, coalescing droplets (especially at the microscale), synchronizing droplet formation, aligning molecules within the droplet, etc. In some cases, the droplets and/or the fluidic streams may include colloids, cells, therapeutic agents, and the like.

RELATED APPLICATIONS

This application is a continuation of PCT International Application No.PCT/US2004/010903, filed Apr. 9, 2004, entitled “Formation and Controlof Fluidic Species” which claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/461,954, filed Apr. 10, 2003, entitled“Formation and Control of Fluidic Species,” by Link, et al.

GOVERNMENT FUNDING

This invention was made with government support under grant numberDMR-0243715 and DMR-0213805 awarded by the National Science Foundationand under grant number NAG3-2284 awarded by NASA. The government hascertain rights in the invention.

FIELD OF INVENTION

The present invention generally relates to systems and methods for theformation and/or control of fluidic species, and articles produced bysuch systems and methods.

BACKGROUND

The manipulation of fluids to form fluid streams of desiredconfiguration, discontinuous fluid streams, droplets, particles,dispersions, etc., for purposes of fluid delivery, product manufacture,analysis, and the like, is a relatively well-studied art. For example,highly monodisperse gas bubbles, less than 100 microns in diameter, havebeen produced using a technique referred to as capillary flow focusing.In this technique, gas is forced out of a capillary tube into a bath ofliquid, where the tube is positioned above a small orifice, and thecontraction flow of the external liquid through this orifice focuses thegas into a thin jet which subsequently breaks into equal-sized bubblesvia a capillary instability. A similar arrangement can be used toproduce liquid droplets in air.

Microfluidic systems have been described in a variety of contexts,typically in the context of miniaturized laboratory (e.g., clinical)analysis. Other uses have been described as well. For example,International Patent Application No. PCT/US01/17246, filed May 25, 2001,entitled “Patterning of Surfaces Utilizing Microfluidic Stamps IncludingThree-Dimensionally Arrayed Channel Networks,” by Anderson, et al.,published as Publication No. WO 01/89788 on Nov. 29, 2001, describesmulti-level microfluidic systems that can be used to provide patterns ofmaterials, such as biological materials and cells, on surfaces. Otherpublications describe microfluidic systems including valves, switches,and other components.

While significant advances have been made in dynamics at the macro- ormicrofluidic scale, improved techniques and the results of thesetechniques are still needed.

SUMMARY OF INVENTION

The present invention generally relates to systems and methods for theformation and/or control of fluidic species, and articles produced bysuch systems and methods. The subject matter of this applicationinvolves, in some cases, interrelated products, alternative solutions toa particular problem, and/or a plurality of different uses of a singlesystem or article.

One aspect of the present invention includes a method. The method, inone set of embodiments, includes a step of surrounding a first fluidwith a second fluid by action, at least in part, of an electric field.The method, according to another set of embodiments, includes a step ofreducing a cross-sectional dimension of a fluidic stream exiting anoutlet of a channel using an electric field. In one set of embodiments,the method includes a step of encapsulating a cell in a fluid droplet.

In one set of embodiments, the method includes a step of producing afluidic stream, comprising a first fluid surrounded by a second, liquidfluid, by expelling the first fluid from an outlet of a channel. Thefluidic stream, in some cases, has an average cross-sectional dimensionperpendicular to its direction of travel that is smaller than an averagecross-sectional dimension of the outlet of the channel. In another setof embodiments, the method is defined, at least in part, by a step ofproducing a fluidic stream of droplets. In some cases, the droplets canhave an average diameter of less than about 60 microns and adistribution of diameters such that no more than about 5% of thedroplets have a diameter greater than about 10% of the average diameter.

The method, in another set of embodiments, includes a step of producinga fluid Taylor cone within a liquid. The method, according to yetanother set of embodiments, includes a step of producing a first fluidTaylor cone and a second fluid Taylor cone within the first fluid Taylorcone. In still another set of embodiments, the method includes steps ofproducing a fluid Taylor cone, and producing a tube of fluid from thefluid Taylor cone. In yet another set of embodiments, the methodincludes a step of producing a Taylor cone comprising a fluid having aviscosity of at least about 1.2 cp. In one set of embodiments, themethod is defined, at least in part, by a step of producing a Taylorcone comprising air.

The method, in another set of embodiments, includes steps of charging afluid droplet with an electric charge, and placing the fluid droplet ina solution comprising a species having an opposite electric charge.According to yet another set of embodiments, the method is defined, atleast in part, by a step of producing an entity comprising a fluid inthe presence of an electric field, where the fluid comprisessubstantially aligned molecules, where, in the absence of the electricfield, the fluid does not comprise substantially aligned molecules.

In still another set of embodiments, the method includes steps ofproducing a first set of substantially parallel microtubes, producing asecond set of substantially parallel microtubes, and contacting thefirst set of microtubes and the second set of microtubes.

In one set of embodiments, the method includes a step of producing, in acarrying liquid, a first droplet comprising a first liquid, where thefirst droplet has a maximum cross-sectional dimension of less than about100 microns. The method also includes a step of producing a seconddroplet comprising a second liquid, where the second droplet has amaximum cross-sectional dimension of less than about 100 microns. Insome cases, the method further includes a step of allowing the firstdroplet and the second droplet to coalesce into one combined droplet.

In another set of embodiments, the method includes a step of producing,in a microfluidic channel, a first droplet comprising a first liquid,where the first droplet is surrounded by a carrying liquid. The methodmay also include steps of producing a second droplet comprising a secondliquid, and allowing the first droplet and the second droplet tocoalesce into one combined droplet.

The method, according to yet another set of embodiments, may be defined,at least in part, by steps of imparting an electric charge to a firstdroplet comprising a first fluid and an opposite electric charge to asecond droplet comprising a second fluid, and allowing the first andsecond droplets to coalesce into one combined droplet due to mutualcharge attraction, under conditions in which the first and seconddroplets would not coalesce absent the respective electric chargesimparted.

Another aspect of the present invention involves an apparatus. In oneset of embodiments, the apparatus includes a channel, a reduction systemconstructed and arranged to reduce an average cross-sectional dimensionof a fluidic stream exiting an outlet of the channel, and a secondchannel constructed and arranged to surround at least a portion of thefluidic stream with a second, liquid fluid. In another set ofembodiments, the apparatus includes a channel having an outlet, and anelectric field generator constructed and arranged to reduce across-sectional dimension of a fluid exiting the outlet of the channel.

In one set of embodiments, the apparatus includes a channel having anoutlet through which a fluid can be expelled. In some cases, the channelis positioned to allow the fluid to be directed at a reduction system,where the reduction system is constructed and arranged to produce, fromthe fluid, a fluidic stream of droplets having an average diameter ofless than about 60 microns and a distribution of diameters such that nomore than about 5% of the droplets have a diameter greater than about10% of the average diameter.

The apparatus, according to another set of embodiments, includes a firstchannel having an outlet for expelling a first fluid, a second channelhaving an outlet for expelling a second fluid, and an electric fieldgenerator positioned to generate an electric field proximate thepredetermined location. In certain embodiments, the second channel ispositioned to allow the second fluid to surround at least a portion ofthe first fluid at a predetermined location.

According to yet another set of embodiments, the apparatus comprises achannel having an outlet, an electric field generator positioned toinduce an electric field in proximity of the outlet of the channel, anda dimensional restriction having a cross-sectional dimension smallerthan an average cross-sectional dimension of the outlet of the channel.In some cases, the dimensional restriction is in fluidic communicationwith the channel.

In one set of embodiments, the apparatus includes a channel having anoutlet, an electric field generator positioned to create a Taylor conein a fluid exiting the outlet of the channel, and a stabilization systemable to restrict motion of the Taylor cone. The apparatus, in anotherset of embodiments, includes a first channel having an outlet, a secondchannel having an outlet, and an electric field generator positioned tocreate a first Taylor cone in a first fluid exiting the outlet of thefirst channel and a second Taylor cone in a second fluid exiting theoutlet of the second channel.

The invention, according to yet another aspect, is an article. In oneset of embodiments, the article includes an emulsion of at least a fluidin a liquid. In some cases, the fluid consists essentially of dropletshaving an average diameter of less than about 1 micron and adistribution of diameters such that no more than about 5% of thedroplets have a diameter greater than about 10% of the average diameter.The article, according to another set of embodiments, includes anemulsion comprising charged droplets in a liquid. The droplets may beessentially saturated in charge in some cases.

In one set of embodiments, the article includes an entity comprising atleast a first fluid, where the first fluid comprises moleculessubstantially aligned in an electric field. The article, in another setof embodiments, includes a liquid comprising a droplet having across-sectional dimension of less than about 100 microns, where thedroplet comprises a first fluid and a second fluid immiscible with thefirst fluid. According to yet another set of embodiments, the article isdefined, at least in part, by an ordered array of microtubes.

In one aspect, the present invention is directed to a method of makingone or more of the embodiments described herein. In another aspect, thepresent invention is directed to a method of using one or more of theembodiments described herein. In yet another aspect, the presentinvention is directed to a method of promoting one or more of theembodiments described herein.

Other advantages and novel features of the invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting disclosure, thepresent specification shall control.

BRIEF DESCRIPTION OF DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For the purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1A-1F illustrate various fluidic streams flowing from channeloutlets, some of which have a cross-sectional dimension smaller than across-sectional dimension of the channel outlet, in accordance with oneembodiment of the invention;

FIGS. 2A-2D illustrate certain systems for reducing the cross-sectionaldimension of a fluidic stream produced from a channel outlet, in variousembodiments of the invention;

FIGS. 3A-3D illustrate various nested Taylor cones, according to certainembodiments of the invention;

FIGS. 4A-4C illustrate various fluidic stream arrays, according to someembodiments of the invention;

FIGS. 5A-5G illustrate uncharged and charged droplets in channels,according to certain embodiments of the invention;

FIGS. 6A-6D illustrate bridge formation, in accordance with anotherembodiment of the invention;

FIGS. 7A-7D illustrate droplets having two or more fluid regions,according to one embodiment of the invention;

FIG. 8 illustrates droplet coalescence, according to another embodimentof the invention;

FIGS. 9A-9H illustrate electrical interactions for droplet formation, inaccordance with certain embodiments of the invention;

FIGS. 10A-10C illustrate articles that can be produced using certainembodiments of the invention; and

FIGS. 11A-11I illustrate certain systems for reducing thecross-sectional dimension of a fluidic stream produced from a channeloutlet, in various embodiments of the invention.

DETAILED DESCRIPTION Definitions

The following definitions will aid in the understanding of theinvention. As used herein, the term “fluid” generally refers to asubstance that tends to flow and to conform to the outline of itscontainer. Typically, fluids are materials that are unable to withstanda static shear stress. When a shear stress is applied to a fluid, itexperiences a continuing and permanent distortion. Typical fluidsinclude liquids (e.g., water or aqueous solutions) and gases (e.g., air,O₂, N₂, etc.), but may also include free flowing solid particles,viscoelastic fluids, and the like. The fluid may have any suitableviscosity, for example, a viscosity similar to water (e.g., as in anaqueous solution), oil, etc. In certain embodiments of the invention,the liquid may include an oil or an organic solvent, such as those knownto ordinary skill in the art. If two or more fluids are present, eachfluid may be independently selected among essentially any fluids(liquids, gases, and the like) by those of ordinary skill in the art, byconsidering the relationship between the fluids. The fluids may each bemiscible or immiscible. For example, two fluids can be selected to beimmiscible within the time frame of formation of a stream of fluids, orwithin the time frame of reaction or interaction. As an example, wherethe portions remain liquid for a significant period of time, the fluidsmay be immiscible. As another example, where, after contact and/orformation, the dispersed portions are quickly hardened by polymerizationor the like, the fluids need not be as immiscible. Those of ordinaryskill in the art can select suitable miscible or immiscible fluids,using contact angle measurements or the like, to carry out thetechniques of the invention.

As used herein, the term “fluid stream” or “fluidic stream” refers tothe flow of a fluid, typically generally in a specific direction. Thefluidic stream may be continuous and/or discontinuous. A “continuous”fluidic stream is a fluidic stream that is produced as a single entity,e.g., if a continuous fluidic stream is produced from a channel, thefluidic stream, after production, appears to be contiguous with thechannel outlet. An example of a continuous fluidic stream is illustratedin FIG. 1B. The continuous fluidic stream may be laminar, or turbulentin some cases. The continuous fluidic stream may be, e.g., solid orhollow (i.e., containing a second fluid internally, for example, as in ahollow tube). It is to be understood that wherever “tube” is usedherein, the structure can be a hollow, a solid or filled (i.e., nothollow) stream, a stream that includes a central core and a surroundinglayer or layers, any of which can be selectively reacted with anyothers, or solidified, or the like. In some cases, the central core ishollow, and/or fluid may be removed from a hardened surrounding fluid toproduce a hollow tube.

Similarly, a “discontinuous” fluidic stream is a fluidic stream that isnot produced as a single entity. A discontinuous fluidic stream may havethe appearance of individual droplets, optionally surrounded by a secondfluid. A “droplet,” as used herein, is an isolated portion of a firstfluid that completely surrounded by a second fluid. In some cases, thedroplets may be spherical or substantially spherical; however, in othercases, the droplets may be non-spherical, for example, the droplets mayhave the appearance of “blobs” or other irregular shapes, for instance,depending on the external environment. An example of a discontinuousfluidic stream is illustrated in FIG. 1A. As used herein, a first entityis “surrounded” by a second entity if a closed loop can be drawn oridealized around the first entity through only the second entity. Afirst entity is “completely surrounded” if closed loops going throughonly the second entity can be drawn around the first entity regardlessof direction.

In certain cases, a fluidic stream may be produced on the microscale,for example, in a microchannel. Thus, in some, but not all embodiments,at least some of the components of the systems and methods describedherein using terms such as “microfluidic” or “microscale.” As usedherein, “microfluidic,” “microscopic,” “microscale,” the “micro-” prefix(for example, as in “microchannel”), and the like generally refers toelements or articles having widths or diameters of less than about 1 mm,and less than about 100 microns (micrometers) in some cases. In somecases, the element or article includes a channel through which a fluidcan flow. In all embodiments, specified widths can be a smallest width(i.e. a width as specified where, at that location, the article can havea larger width in a different dimension), or a largest width (i.e.where, at that location, the article has a width that is no wider thanas specified, but can have a length that is greater). Additionally,“microfluidic,” as used herein, refers to a device, apparatus or systemthat includes at least one microscale channel.

A “channel,” as used herein, means a feature on or in an article (e.g.,a substrate) that at least partially directs the flow of a fluid. Insome cases, the channel may be formed, at least in part, by a singlecomponent, e.g. an etched substrate or molded unit. The channel can haveany cross-sectional shape, for example, circular, oval, triangular,irregular, square or rectangular (having any aspect ratio), or the like,and can be covered or uncovered (i.e., open to the external environmentsurrounding the channel). In embodiments where the channel is completelycovered, at least one portion of the channel can have a cross-sectionthat is completely enclosed, and/or the entire channel may be completelyenclosed along its entire length with the exception of its inlet andoutlet.

A channel may have an aspect ratio (length to average cross-sectionaldimension) of at least 2:1, more typically at least 3:1, 5:1, or 10:1.As used herein, a “cross-sectional dimension,” in reference to a fluidicor microfluidic channel, is measured in a direction generallyperpendicular to fluid flow within the channel. An open channelgenerally will include characteristics that facilitate control overfluid transport, e.g., structural characteristics (an elongatedindentation) and/or physical or chemical characteristics (hydrophobicityvs. hydrophilicity) and/or other characteristics that can exert a force(e.g., a containing force) on a fluid. The fluid within the channel maypartially or completely fill the channel. In some cases the fluid may beheld or confined within the channel or a portion of the channel in somefashion, for example, using surface tension (e.g., such that the fluidis held within the channel within a meniscus, such as a concave orconvex meniscus). In an article or substrate, some (or all) of thechannels may be of a particular size or less, for example, having alargest dimension perpendicular to fluid flow of less than about 5 mm,less than about 2 mm, less than about 1 mm, less than about 500 microns,less than about 200 microns, less than about 100 microns, less thanabout 60 microns, less than about 50 microns, less than about 40microns, less than about 30 microns, less than about 25 microns, lessthan about 10 microns, less than about 3 microns, less than about 1micron, less than about 300 nm, less than about 100 nm, less than about30 nm, or less than about 10 nm or less in some cases. In oneembodiment, the channel is a capillary. Of course, in some cases, largerchannels, tubes, etc. can be used to store fluids in bulk and/or delivera fluid to the channel.

In some embodiments, the dimensions of the channel may be chosen suchthat fluid is able to freely flow through the channel, for example, ifthe fluid contains cells. The dimensions of the channel may also bechosen, for example, to allow a certain volumetric or linear flowrate offluid in the channel. Of course, the number of channels and the shape ofthe channels can be varied by any method known to those of ordinaryskill in the art. In some cases, more than one channel or capillary maybe used. For example, two or more channels may be used, where they arepositioned inside each other, positioned adjacent to each other, etc.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

As used herein, “or” should be understood to mean inclusively or, i.e.,the inclusion of at least one, but including more than one, of a numberor list of elements. Only terms clearly indicated to the contrary, suchas “only one of” or “exactly one of,” will refer to the inclusion ofexactly one element of a number or list of elements.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements that the phrase “at least one” refers to, whether related orunrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

The following documents are incorporated herein by reference in theirentirety: U.S. Provisional Patent Application Ser. No. 60/392,195, filedJun. 28, 2002, entitled “Multiphase Microfluidic System and Method,” byStone, et al.; U.S. Provisional Patent Application Ser. No. 60/424,042,filed Nov. 5, 2002, entitled “Method and Apparatus for FluidDispersion,” by Link, et al.; U.S. Provisional Patent Application Ser.No. 60/461,954, filed Apr. 10, 2003, entitled “Formation and Control ofFluidic Species,” by Link, et al.; U.S. Provisional Patent ApplicationSer. No. 60/498,091, filed Aug. 27, 2003, entitled “Electronic Controlof Fluidic Species,” by Link, et al.; U.S. patent application Ser. No.08/131,841, filed Oct. 4, 1993, entitled “Formation of MicrostampedPatterns on Surfaces and Derivative Articles,” by Kumar, et al., nowU.S. Pat. No. 5,512,131, issued Apr. 30, 1996; International PatentApplication No. PCT/US96/03073, filed Mar. 1, 1996, entitled“Microcontact Printing on Surfaces and Derivative Articles,” byWhitesides, et al., published as WO 96/29629 on Jun. 26, 1996; U.S.patent application Ser. No. 09/004,583, filed Jan. 8, 1998, entitled“Method of Forming Articles Including Waveguides via CapillaryMicromolding and Microtransfer Molding,” by Kim, et al., now U.S. Pat.No. 6,355,198, issued Mar. 12, 2002; International Patent ApplicationNo. PCT/US01/17246, filed May 25, 2001, entitled “Patterning of SurfacesUtilizing Microfluidic Stamps Including Three-Dimensionally ArrayedChannel Networks,” by Anderson, et al., published as Publication No. WO01/89788 on Nov. 29, 2001; International Patent Application No.PCT/US01/46181, filed May 25, 2001, entitled “Methods and Compositionsfor Encapsulating Active Agents,” by Weitz, et al., published asPublication No. WO 02/47665 on Jun. 20, 2002; International PatentApplication No. PCT/US02/23462, filed Jul. 24, 2002, entitled “LaminarMixing Apparatus and Methods,” by Stroock, et al., published as WO03/011443 on Feb. 13, 2003; and International Patent Application No.PCT/US03/20542, filed Jun. 30, 2003, entitled “Method and Apparatus forFluid Dispersion,” by Stone, et al., published as Publication No. WO2004/002627 on Jan. 8, 2004.

EMBODIMENTS

The present invention generally relates to systems and methods for theformation and/or control of fluidic species, and articles produced bysuch systems and methods. The invention involves unique fluid channels,systems, controls, and/or restrictions, and combinations thereof, asfurther described below, as well as other features. In certainembodiments, the invention allows fluidic streams (which can becontinuous or discontinuous, i.e., droplets) to be formed and/orcombined, at a variety of scales, including microfluidic scales. In oneset of embodiments, a fluidic stream is produced from a channel, where across-sectional dimension of the fluidic stream is smaller than that ofthe channel, for example, through the use of structural elements, otherfluids, and/or applied external fields, etc. In some cases, a Taylorcone may be produced. In another set of embodiments, a fluidic streammay be manipulated in some fashion, for example, to create tubes (whichmay be hollow or solid), droplets, nested tubes or droplets, arrays oftubes or droplets, meshes of tubes, etc. In some cases, dropletsproduced using certain embodiments of the invention may be charged orsubstantially charged, which may allow their further manipulation, forinstance, using applied external fields. Non-limiting examples of suchmanipulations include producing charged droplets, coalescing droplets(especially at the microscale), synchronizing droplet formation,aligning molecules within the droplet, etc. In some cases, the dropletsand/or the fluidic streams may include colloids, cells, therapeuticagents, or the like.

Examples of methods of forming such articles and systems are furtherdescribed below. Typically, when a fluidic stream is formed by producingor expelling fluid from a channel, the diameter or other cross-sectionaldimension of the droplet is larger than the diameter or othercross-sectional dimension of the channel outlet. The fluidic stream maybe, for instance, a continuous stream of fluid (e.g., a “tube”) or adiscontinuous stream of fluid (e.g., droplets); for example, as is shownin FIGS. 1A-1B, a fluid 15, produced from channel 12 through channeloutlet 13, forms a discontinuous stream of fluid (FIG. 1A) or acontinuous stream of fluid (FIG. 1B).

In one aspect, however, the present invention relates to the productionor expulsion of a fluidic stream from a channel where the fluidic streamhas a cross-sectional dimension that is smaller than a cross-sectionaldimension of the channel outlet. In some cases, the present inventionallows the production or expulsion of a fluidic stream from a channel tooccur in a manner that provides unique control over the fluidic streamand/or unique combinations of fluid or materials, as further describedherein. As an example, a fluidic stream may be manipulated using one ormore structural elements in or near its path of flow. As anotherexample, a fluidic stream being produced or expelled from the channelmay be contacted with another fluid in some fashion to manipulate thefluidic stream. As yet another example, an externally applied field(e.g., an electric and/or a magnetic field) may be generated proximatethe channel outlet and/or proximate a fluidic stream to manipulate thefluidic stream. Combinations of any of these and/or other systems andtechniques, e.g. as further described herein, are also contemplated inthe present invention. Furthermore, the size of the fluidic stream,including droplet sizes in discontinuous streams, can be very preciselycontrolled in some instances.

In some cases, the fluidic stream may have an average cross-sectionaldimension smaller than about 90% of an average cross-sectional dimensionof the channel, and in certain embodiments, smaller than about 80%,about 70%, about 60%, about 50%, about 40%, or about 30% of the averagecross-sectional dimension of the channel. In other embodiments, thefluidic stream may have an average cross-sectional dimension smallerthan about 20%, about 10%, about 5%, about 3%, about 1%, about 0.5%,about 0.3%, about 0.1%, about 0.05%, about 0.03%, or about 0.01% of theaverage cross-sectional dimension of the channel. The fluidic stream, insome embodiments, may be produced on the microscale, e.g., using amicrofluidic channel. For instance, the fluidic stream may have anaverage cross-sectional dimension of less than about 1 mm, less thanabout 500 microns, less than about 300 microns, or less than about 100microns. In some cases, the fluidic stream may have an average diameterof less than about 60 microns, less than about 50 microns, less thanabout 40 microns, less than about 30 microns, less than about 25microns, less than about 10 microns, less than about 5 microns, lessthan about 3 microns, or less than about 1 micron.

In one set of embodiments, a structural element may be used tomanipulate the fluidic stream in some fashion to produce a fluidicstream that has a cross-sectional dimension that is smaller than across-sectional dimension of a channel outlet that produces the fluid.In some cases, a fluidic stream may be produced where no cross-sectionaldimension of the fluidic stream has a dimension that is larger than thesmallest cross-sectional dimension of the channel outlet. A “structuralelement,” as used herein, is a physical feature, in or proximate thechannel, that is able to at least partially alter fluid flow from thechannel. Examples of structural elements include dimensionalrestrictions, ridges, grooves, or the like. As used herein, a“dimensional restriction” is a structural element that is shaped toreduce a cross-sectional dimension of the fluidic stream. In some cases,the dimensional restriction is an annular orifice, but it can also takeany of a varieties of forms, for example, elongate, ovoid, square,triangular, or the like. The dimensional restriction is non-valved inpreferred embodiments. That is, the dimensional restriction is anorifice that cannot be switched between an open state and a closedstate, and is typically of fixed size. As a non-limiting example, inFIG. 1C, a fluid 15 that is expelled from a channel 12 passes through adimensional restriction 14 that causes the fluid to form a stream havinga cross-sectional dimension 10 smaller than a cross-sectional dimensionof the channel outlet 13. As another example, the fluid, after passingthrough the dimensional restriction, may become a discontinuous streamof fluid (not shown). Other non-limiting examples of dimensionalrestrictions may be seen in FIGS. 2B-2D. Still other examples ofdimensional restrictions can be seen in International Patent ApplicationNo. PCT/US03/20542, filed Jun. 30, 2003, entitled “Method and Apparatusfor Fluid Dispersion,” by Stone, et al., incorporated herein byreference.

In some cases, the fluidic stream and/or the surrounding fluid has amean cross-sectional dimension no smaller than 90% of the averagecross-sectional dimension of the dimensional restriction, and in otherembodiments, no smaller than 80%, 70%, 60%, 50%, 40%, or 30% of theaverage cross-sectional dimension of the dimensional restriction. Thiscan be advantageous in certain cases in that a system of the inventioncan be operated over a range of fluid flowrates, and still produce afluidic stream having the same, or approximately the same, size orcross-sectional dimension.

In another set of embodiments, one or more additional fluidic streamsmay be used to manipulate the fluidic stream in some fashion to producea fluidic stream that has a cross-sectional dimension that is smallerthan a cross-sectional dimension of a channel outlet that produces thefluid. The second fluid may be directed at the fluid and/or at thechannel in such a way as to cause the fluidic stream produced by thechannel to have a cross-sectional dimension smaller than across-sectional dimension of a channel outlet, and in some cases, suchthat no cross-sectional dimension of the fluidic stream has a dimensionthat is larger than the smallest cross-sectional dimension of thechannel. In one embodiment, an additional fluid or fluids are directedin such a way as to surround or “sheath” the fluid being produced by thechannel, reducing a cross-sectional dimension of the fluidic stream. Theinvention, in some cases, thus involves control over the averagecross-sectional dimensions of the fluidic stream by control of theflowrate of a sheathing fluid, and/or control of the ratios of theflowrate of the fluidic stream relative to the sheathing fluid.

One non-limiting example is shown in FIG. 1D. In this figure, a fluid 15that is expelled from a channel 12 is surrounded by a sheathing fluid 16that flows around channel 12. The flow of fluid 16 around channel 12 mayrestrain the size of the fluidic stream 15, for example, producing acontinuous (shown) or discontinuous fluidic stream. In another example(not shown), two or more sheathing fluidic streams may be provided, forexample, on different sides of a fluidic stream, to reduce across-sectional dimension of the fluidic stream.

In yet another set of embodiments, an externally applied field (e.g., anelectric and/or a magnetic field) may be generated proximate the channeloutlet and/or proximate a fluidic stream to manipulate the fluidicstream, for example, to produce a fluidic stream that has across-sectional dimension that is smaller than a cross-sectionaldimension of a channel outlet that produces the fluid. In oneembodiment, the externally applied field includes a magnetic field.Techniques for producing suitable magnetic fields are known to those ofordinary skill in the art, for example, through the use of permanentmagnets, electromagnets, or the like. In another embodiment, theexternally applied field includes an electric field. The electric fieldmay be generated from an electric field generator, i.e., a system ableto produce an electric field, for example, directed substantially at thechannel or at the channel outlet, and/or directed proximate the fluidicstream exiting the channel outlet. Techniques for producing a suitableelectric field are known to those of ordinary skill in the art. Forexample, an electric field may be produced by applying a voltage dropacross electrodes positioned proximate the channel outlet and/or fluidicstream. The electrodes can be fashioned from any suitable electrodematerial, for example, as silver, gold, copper, carbon, platinum,copper, tungsten, tin, cadmium, nickel, indium tin oxide (“ITO”), etc.,as is known to those of ordinary skill in the art. In some cases,transparent or substantially transparent electrodes may be used.

A non-limiting example of the use of an externally applied field isshown in FIG. 1E. In this figure, a fluid 15 that is expelled from achannel 12 is substantially electrically conductive and a second fluid17 surrounding channel 12 is substantially non-electrically conductive.As used herein, a material is “conductive” if it is able to conductelectricity, e.g., a conductor or a semiconductor. When an externallyapplied electric field is applied, a net electrical charge may build upon the surface 19 of the fluid at the channel outlet 13 due to thepresence of an applied inductive electric field (illustrated in FIG. 1Eas “+”). Due to charge repulsion, the shape of fluid 15 at the channeloutlet 13 may be extended in some cases to form a substantiallycone-like shape, known as a “Taylor cone” 11. A fluidic stream 15 (e.g.,droplets or tubes) may be produced from the end of Taylor cone 11, whichmay allow excess charge to be removed from the Taylor cone 11.

Thus, an aspect of the present invention relates to the application ofan electric field proximate a fluid exiting an outlet of a channel toform a Taylor cone, or other shape having reduced averagecross-sectional dimension, relative to the average cross-sectionaldimension of the outlet. In some cases, the fluid exiting the channelmay be brought into contact with another fluid, which can be a liquid ora gas, and which may surround or at least partially surround the fluidexiting the channel. The fluid surrounding the fluidic stream may be airor other gas, or in some cases, a liquid. As an example, the fluid maycompletely surround the fluid exiting the channel in embodiments where afluidic stream (e.g., droplets, a tube, etc.) is formed by the fluidexiting the channel. In other cases, however, there may be no fluidsurrounding the fluidic stream, i.e., the Taylor cone may be formed in avacuum.

A “Taylor cone,” as used herein, is a shape that a fluidic stream of anat least partially electrically conductive fluid assumes when exposed toan externally applied inductive electric field, as is known to those ofordinary skill in the art. In the formation of a Taylor cone, anelectric field may be applied to a fluidic stream exiting the outlet ofa channel so as to pass through the fluid in the general direction offluid flow. The fluid may assume a surface charge which is susceptibleto the electric field, and the electric field thereby applies anattractive force to the fluid in the direction of fluid flow, thusforming an approximate cone shape with a cross-sectional dimension ofthe fluidic stream decreasing in the direction of fluid flow until itreaches a small cross-sectional dimension.

In embodiments where a fluidic stream is formed from a Taylor cone, thefluidic stream may be continuous (e.g., a “tube”) or discontinuous(e.g., the fluidic stream may break up into droplets). In some cases,the continuity of the fluidic stream may be controlled by controllingthe viscosity of the fluid forming the Taylor cone, and/or thesurrounding fluid (if present). Higher viscosity fluids may more readilyform continuous fluids from Taylor cones. For example, if a fluid withina channel is relatively viscous (e.g., greater than the viscosity ofwater), for instance, a fluid having a viscosity of at least about 1.2cp, at least about 2 cp, at least about 5 cp, at least about 10 cp, atleast about 50 cp, or at least about 100 cp, then a continuous stream or“tube” of fluid 65 may form from the end of the Taylor cone, e.g., as isshown in FIG. 1F, where a continuous fluidic stream 15 is produced froma Taylor cone 11. Non-limiting examples of such viscous fluids includean organic fluid, such as an oil or hydrocarbon.

As a specific example, in FIG. 1E, a Taylor cone may be created byapplying an external electric field to a fluidic stream 15 exiting achannel 12 through channel outlet 13. As described above, electriccharge may build on the surface 19 of the Taylor cone 11. Droplets 15that form from the end of the Taylor cone due to fluid flow may behighly charged or even essentially saturated in charge in some cases(i.e., where additional charge cannot be added to the droplet withoutcausing the droplet to dissociate into separate droplets). Once acharged droplet is formed, the charged droplet may repel other chargeddroplets due to electrical charge interaction. This interaction maycause the droplet to move from the Taylor cone at an accelerated speed,and/or cause the Taylor cone to substantially deform or move (“whippingbehavior”) through electric field interactions between the chargedsurface of the Taylor cone and the charged droplet. This interaction maybe hard to predict and/or model in some instances.

Thus, in some cases, the present invention allows for very preciselycontrolled fluidic stream formation (e.g., droplets or a tube), forexample, by controlling and/or limiting the movement of the Taylor coneand the production of a fluidic stream from the Taylor cone, and/or bycontrolling the characteristics of the fluid surrounding the Taylor cone(e.g., velocity, flowrate, viscosity, etc.). The present invention, inone embodiment, allows a fluidic stream to be produced that has across-sectional dimension smaller than a cross-sectional dimension ofthe channel outlet producing a Taylor cone. For example, the fluidicstream may have a cross-sectional dimension smaller than about 90% of anaverage cross-sectional dimension of the channel, and in certainembodiments, smaller than about 80%, about 70%, about 60%, about 50%,about 40%, about 30%, about 20%, about 10%, about 5%, about 3%, about1%, about 0.5%, about 0.3%, about 0.1%, about 0.05%, about 0.03%, orabout 0.01% of the average cross-sectional dimension of the channel,e.g., as previously described. In cases where the fluidic stream formsdroplets, the droplets may have a homogenous distribution of diameters,i.e., the droplets may have a distribution of diameters in some cases,such that no more than about 10%, about 5%, about 3%, about 1%, about0.03%, or about 0.01% of the droplets have an average diameter greaterthan about 10%, about 5%, about 3%, about 1%, about 0.03%, or about0.01% of the average diameter of the droplets.

Thus, the present invention provides systems and methods for stabilizinga Taylor cone, according to one set of embodiments. For instance, aTaylor cone produced by a fluidic stream exiting channel in the presenceof an electric field may be stabilized by reducing the cross-sectionaldimension of the fluidic stream such that it is smaller than across-sectional dimension of the channel outlet, i.e., preventing orlimiting the Taylor cone from substantially moving when a chargeddroplet is produced from the Taylor cone. As used herein, the Taylorcone is prevented from “substantially moving” if a charged droplet,formed after separation of the droplet from the Taylor cone, does notalter the cone angle of the Taylor cone by more than about 50%, and insome cases, such that the cone angle is not altered by more than about40%, about 30%, about 20% or about 10%. Methods of reducing thecross-sectional dimension of the fluidic stream include those previouslydescribed, for example, using one or more additional fluidic streams(e.g., a sheathing fluid) and/or structural elements such as dimensionalrestriction.

For instance, in one embodiment, the invention includes one or moresheathing fluids able to limit the motion of the Taylor cone, forexample, due to differences in viscosity between the Taylor cone fluidand the sheathing fluid, and/or due to the flowrate of the sheathingfluid. For example, a sheathing fluid may be highly viscous relative tothe Taylor cone fluid, thus at least partially restraining movement ofthe Taylor cone during charged droplet formation. As another example,illustrated in FIG. 2A, a sheathing fluid may be moving at a highflowrate, relative to a Taylor cone fluid, such that the sheathing fluidis able to at least partially restrain movement of the Taylor coneduring charged droplet formation.

In another embodiment, the Taylor cone may be stabilized by urging atleast a portion of the Taylor cone and/or a fluidic stream formed fromthe Taylor cone (which may be continuous or discontinuous) to flowthrough a dimensional restriction downstream of the outlet. For example,the channel outlet may feed fluid into an enclosed space, from which thefluid exits through a dimensional restriction. Non-limiting examples ofsuch methods of stabilizing Taylor cones are illustrated in FIGS. 2B-2D,where Taylor cone 11 formed from channel 12 at channel outlet 13 isstabilized by dimensional restriction 14. Fluid flowrates may increaseas the fluid passes through the dimensional restriction, which mayconstrain the movement of the Taylor cone within the dimensionalrestriction. Thus, sheathing fluid 16, as it passes through thedimensional restriction, may at least partially restraining movement ofthe Taylor cone. In FIGS. 2B-2D, the dimensional restriction is anannular orifice, but it can also take any of a variety of forms.

In yet another embodiment, the Taylor cone may be at least partiallyrestraining from movement through a mechanical interaction, for example,through mechanical interaction of at least a portion of the Taylor conewith a side of a channel, a structural element such as a dimensionalrestriction, or the like.

Combinations of the above embodiments and/or other embodiments are alsopossible. For example, a Taylor cone may be at least partiallystabilized by at least partially surrounding the Taylor cone fluid witha sheathing fluid and urging at least a portion of the Taylor cone topass through a dimensional restriction. In some cases, the sheathingfluid may have an average cross-sectional dimension no smaller thanabout 90% of an average cross-sectional dimension of the dimensionalrestriction, and in certain embodiments, no smaller than about 80%,about 70%, about 60%, about 50%, about 40%, or about 30% of the averagecross-sectional dimension of the dimensional restriction.

According to one set of embodiments, more than one Taylor cone may beproduced using the systems and methods of the invention, for example,nested Taylor cones and/or parallel Taylor cones, etc. In some cases,each of the Taylor cones may independently be used to produce streamsthat are continuous (e.g., hollow or filled) or discontinuous (e.g.,droplets), such as those previously described. In certain embodiments,the droplets may be highly charged or essentially saturated in charge.

One embodiment of the invention allows for the production of nestedTaylor cones, i.e., where one Taylor cone at least partially surroundsanother Taylor cone. Non-limiting examples of nested Taylor cones areshown in FIGS. 3A-3D. Thus, in some cases, the invention may include atleast a first channel for carrying a first fluid and a second channelfor carrying a second fluid, where the channels are positioned such thatone Taylor cone at least partially surrounds another Taylor cone. Incertain instances, the channels are positioned such that one fluidencapsulates a second fluid. The fluids may each be miscible, orimmiscible in some cases, depending on the application. As examples, inFIGS. 3A and 3B, two Taylor cones 33, 34 are produced in two channels31, 32, respectively, that are nested (i.e., where channel 31 is nestedin channel 32). The resulting fluidic stream of droplets (FIG. 3A) or atube (FIG. 3B) include one fluid 35 nested inside a second fluid 36(produced from respective channels 31 and 32). FIGS. 3C and 3Dillustrate similar, non-limiting examples having three Taylor cones andthree fluids 37, 38, and 39. Depending on the configuration of thechannels, a nesting of three fluids may be produced (FIG. 3C), a nestingof two fluids inside a third fluid may be produced (FIG. 3D), etc. Thoseof ordinary skill in the art will be ascertain other embodiments andequivalents; for example, a nesting of four, five, or more Taylor conesmay be used in some cases.

As examples, if one fluid is a gas and one is a liquid, an aerosol maybe produced. As another example, if one fluid is a gas and one fluid isan oil, a foam or a foam-like structure may be produced (for example, ifthe internal contents of the droplet are allowed to mix). As yet anotherexample, a hollow capsule or a hollow tube may be produced, for example,if a liquid is used to encapsulate a gas. In some instances, twomiscible liquids may be mixed if the two fluids are both miscibleliquids. As another example, two reactive components may be mixed orencapsulated, where a reaction (for example, a solidification reaction)occurs at the interface of the two components. Thus, for instance, adroplet or a tube having a solid shell surrounding a liquid center maybe fabricated using the invention. In another example, one fluid of adroplet which may be reactive to the environment that the droplet is inmay be encapsulated by a fluid not substantially reactive to theenvironment; thus, a protective “shell” around the fluid may be formedby the other fluid.

Another embodiment of the invention allows for the production of aseries of two, three, or more parallel Taylor cones (e.g., FIG. 4A),which may allow the production of a series of two, three, or moreparallel continuous fluidic streams in some cases. In cases where atleast one of the fluidic streams is hardenable into a solid (e.g., asfurther described below), the invention also relates to a mesh of tubes.For example, a series of channels may be used to produce a series oftubes, as is shown in FIG. 4A. The tubes may be solid or hollow. Thechannels may have any orientation with respect to each other, e.g.,parallel, perpendicular, at an angle, etc. In some cases, the series oftubes may be bonded with other tubes to create a mesh or a network oftubes, e.g., as is shown in FIG. 4B (illustrating two series of paralleltubes 41, 42) and FIG. 4C (illustrating three series of parallel tubes41, 42, 43). Each series of tubes can be positioned in any orientationwith respect to other tubes, and with any desired spacing. In somecases, the network of tubes may have controlled pore size distributions,where the pores are defined by the spaces between the tubes. Forinstance, a mesh of nanotubes can be created that has a very narrow poresize distribution, e.g., such that less than about 5%, less than about3%, less than about 1%, less than about 0.05%, less than about 0.03%,less than about 0.01%, less than about 0.005%, less than about 0.003%,or less than about 0.001% or less of the pores has a diameter greaterthan the average diameter of all the pores.

Another aspect of the present invention relates to the hardening of afluidic stream into a solid. The fluidic stream may be produced, forexample, from a Taylor cone, from a channel outlet having across-sectional dimension greater than that of the fluidic stream,and/or any system or method described herein for producing fluidicstreams. The fluidic stream may be continuous (e.g., tubes, which may behollow or filled) or discontinuous stream (e.g., droplets), as describedabove. As used herein, the “hardening” of a fluidic stream refers to aprocess by which at least a portion of the fluidic stream is convertedinto a solid or at least a semi-solid state (e.g., a gel, a viscoelasticsolid, etc.).

In one set of embodiments, the fluidic stream may be hardened throughdrying techniques, i.e., a portion of the fluidic stream is dried toproduce a solid. Non-limiting examples of drying techniques includespray drying, exposure to increased temperatures, or the like. Inanother set of embodiments, the fluidic stream may be hardened through achemical reaction, i.e., a fluid component of the fluidic stream isreacted in some fashion (for example, with another component of thefluidic stream, with a reactant in the fluid surrounding the fluidicstream, with light, etc.) to form a solid.

In one set of embodiments, the entire fluidic stream may be solidified;however, in other embodiments, only a portion of the fluidic stream maybe solidified, thus, for example, resulting in a material having a fluidcore surrounded by a solidified shell. In one example arrangement, afirst fluidic stream is formed, surrounded by a second fluidic stream,and the second fluidic stream is selectively hardened, forming asolidified tube or shell surrounding the first fluidic stream. Asanother example, a continuous fluidic stream can be hardened into smallfibers via a temperature change, a chemical reaction, a photochemicalreaction, etc. As yet another example, droplets formed in this way canbe similarly treated, and/or carried to other droplets for combinationand potential chemical reaction to form beads, colloids, quantum dots,or essentially any other solid article or gel obtainable from liquiddroplets.

In some cases, one or more tubes produced by solidifying a continuousfluidic stream may be used as a fiber or a fiber-optic cable. In anotherexample, multiple solidified fluidic streams may be used in a fabric, amesh, or a membrane. Such materials may, in some cases, have controlledpore size distributions, for example, as previously described.

In yet another aspect, the present invention generally relates to themanipulation of fluidic streams, for example, continuous (e.g., tubes)or discontinuous streams of fluid (e.g., droplets) produced by any ofthe techniques described herein, for example, through the use of Taylorcones as previously described. In some cases, the fluidic stream beingmanipulated may be highly charged or essentially saturated in charge insome cases. For example, the process used to create the droplets (forexample, through use of a Taylor cone) may also cause an electricalcharge to be induced onto the droplets. In certain instances where thefluidic stream includes droplets, the droplets being manipulated mayhave a homogenous distribution of diameters, i.e., the droplets may havea distribution of diameters such that no more than about 10%, about 5%,about 3%, about 1%, about 0.03%, or about 0.01% of the droplets have anaverage diameter greater than about 10%, about 5%, about 3%, about 1%,about 0.03%, or about 0.01% of the average diameter of the droplets.

In one set of embodiments, systems and methods are provided that areable to cause two or more droplets (e.g., arising from discontinuousstreams of fluid) to fuse or coalesce into one droplet, for example, incases where the two or more droplets ordinarily are unable to fuse orcoalesce, for example due to composition, surface tension, droplet size,etc. It should be noted that, in some cases, the below descriptions withrespect to droplets can also be applied to tubes and other continuousfluidic streams, for example, by applying a positive charge to a firsttube and a negative charge to a second tube, in the creation of tubescontaining partially separated fluids, etc.

As an example, in microfluidic systems, the surface tension of thedroplets, relative to the size of the droplets may prevent fusion orcoalescence of the droplets from occurring. In one embodiment, twodroplets may be given opposite electrical charges (i.e., positive andnegative charges, not necessarily of the same magnitude), which mayincrease the electrical interaction of the two droplets such that fusionor coalescence of the droplets can occur. Electrical charges (positiveor negative) may be imparted onto droplets through the use of Taylorcones as previously described, or through any other suitable techniquesknown to those of ordinary skill in the art. For instance, an electricfield may be imposed on a reactor containing the droplets, the dropletsmay be passed through a capacitor, a chemical reaction may occur tocause the droplets to become charged, etc.

For example, as is shown schematically in FIG. 5A, uncharged droplets 51and 52 brought into contact with each other are not able to fuse orcoalesce, for instance, due to their size and/or surface tension. Thedroplets, in some cases, may not be able to fuse even if a surfactant isapplied to lower the surface tension of the droplets, for example, dueto their size if the droplets are microfluidic droplets. An examplephotomicrograph of such a microfluidic system is shown in FIG. 5B, wheredroplets 51 and 52 each are water droplets contained within amicrofluidic channel 53, where the droplets arise from water source 57and are carried by a carrier fluid (oil) 54 arising from channels 58.

However, if the droplets are electrically charged with opposite charges(which can be, but are not necessarily of, the same magnitude), thedroplets may become able to fuse or coalesce. For instance, in FIG. 5C,positively charged droplets 55 and negatively charged droplets 56 aredirected at each other such that the electrical interaction of theoppositely charged droplets causes the droplets to fuse or coalesce intofused droplets 57. An example photomicrograph in microfluidic system isshown in FIG. 5D, where droplets 55 and 56 each are water droplets,contained within a carrier fluid (oil) 54 within a microfluidic channel53. A series of a time sequence of photomicrographs of such a system isshown in FIGS. 5E-5G, where droplets 55 and 56 fuse to form droplets 57.

In some cases, the droplets may fuse through the creation of a “bridge”of fluid between the droplets, as is shown sequentially in FIGS. 6A-6C,which may occur due to the opposite charge interaction between fluids 61and 62. An expanded view of the fluidic bridge 65 is shown in FIG. 6D,showing that the surfaces of the fluidic droplets have been merged. Thecreation of the “bridge” of fluid between the droplets may allow thedroplets to exchange material and/or coalesce or fuse into one droplet.

It should be noted that when two or more droplets “coalesce,” perfectmixing of the fluids from each droplet in the resulting droplet does notinstantaneously occur. In some cases, the fluids may not mix, react, orotherwise interact, thus resulting in a fluid droplet where each fluidremains separate or at least partially separate. In other cases, thefluids may each be allowed to mix, react, or otherwise interact witheach other, thus resulting in a mixed or a partially mixed fluiddroplet. In some cases, the coalesced droplets may be contained within acarrying fluid, for example, an oil in the case of aqueous droplets.

In one set of embodiments, a coalesced droplet (or other fluidic stream)may contain two or more regions of fluid, where the fluids remainseparate or at least partially separate. For example, as is shown inFIG. 7A, in channels 70, a coalesced droplet may initially be formed ofa first region of fluid 71 (from droplet 73) and a second region offluid 72 (from droplet 74). The fluid regions may remain as separateregions, thus resulting in a non-uniform fluid droplet 75. In somecases, the fluid regions within the droplet may remain separate (withoutadditional mixing factors) due to the flow of fluid within the droplet.For instance, the droplet may thus exhibit internal“counter-revolutionary” flow, e.g., as is illustrated in FIG. 7B, wherefluid droplet 68 moves in direction 77. Counter-revolutionary flowwithin a droplet may prevent the fluids within the droplet fromsubstantially mixing in some cases.

As one particular example, a coalesced droplet containing a first regionof fluid containing a first dye (i.e., a molecule or molecules that canbe optically determined to contain a color) and a second region of fluidcontaining a second dye (or no dye) may be created such that the firstregion and the second region are visually different. For instance, thefirst region may be white (or transparent) and the second region may beblack; the first region may be cyan, magenta, and/or yellow and thesecond region may be black or white (or transparent); the first regionmay be red, green, and/or blue and the second region may be black orwhite (or transparent), etc. Such coalesced droplet could be used, forexample, in a system where an electric field or other external field maybe applied to the droplets to alter their visual appearance. In somecases, the coalesced droplets may be hardened into particles, e.g. aspreviously described.

However, in another set of embodiments, two or more regions of fluid ina coalesced droplet (or other fluidic stream) may be allowed topartially or completely mix, react, or otherwise interact with eachother. For example, as is shown in FIG. 7C, in channels 70, a coalesceddroplet may initially be formed of a first region of fluid 71 (fromdroplet 73) and a second region of fluid 72 (from droplet 74). The fluidregions can then mix, react, or otherwise interact, eventually forming acoalesced droplet 78 that is partially or completely (i.e.,homogeneously) mixed. Mixing of the regions of fluid within thecoalesced droplet may be allowed to occur through any suitablemechanism, for example unassisted or natural methods, such as throughdiffusion (e.g., through the interface between the two regions offluid), through reaction of the fluids with each other, and/or throughfluid flow within the droplet (i.e., convection). In some embodiments,only a portion or a component of a region of fluid (for example, areactant, as further described below), interacts with other regions offluid (or a portion or a component thereof), e.g., through mixing,reaction, etc.

In other embodiments, however, mixing of the regions of fluid within thecoalesced droplet may be enhanced in some fashion. As one example, thecoalesced droplet may be passed through one or more channel regionswhich cause the droplet to change direction. The change of direction mayalter convection patterns within the droplet, allowing the two fluids tobe mixed, resulting in an at least partially mixed droplet. As anexample, in FIG. 7D, droplet 76 may be passed through one or more bendswithin channel 70, causing the contents of droplet 76 to be at leastpartially mixed, resulting in droplet 79. Each of the bends mayindependently be a perpendicular bend (e.g., as is illustrated in FIG.7D), and/or a bend having any suitable angle, for example, a bend havingan angle of about 15°, about 30°, about 45°, about 60°, about 75°, about90°, about 105°, about 120°, about 135°, about 150°, about 165°, etc. Asanother example, a droplet may pass by one or more features within orproximate a channel (e.g., ridges, posts, protrusions, beams,indentations, dips, etc.) which are able to cause the contents of thedroplet to be at least partially mixed. Non-limiting examples of suchfeatures are disclosed in International Patent Application No.PCT/US02/23462, filed Jul. 24, 2002, entitled “Laminar Mixing Apparatusand Methods,” by Stroock, et al., published as WO 03/011443 on Feb. 13,2003, incorporated herein by reference.

In one set of embodiments, the droplets (or other fluidic stream) beingfused or coalesced may contain reactants (e.g., chemicals, biologicalmolecules, biological entities such as cells, viruses, bacteria, etc.)able to react or otherwise interact with each other. The reactant may bethe fluid comprising the droplet and/or a fluidic region within thedroplet, and/or the reactant may be carried (e.g., dissolved, suspended,etc.) by a fluid within the droplet and/or within a fluidic region ofthe droplet. The reaction may be, for example, a precipitation reaction,i.e., the reactants may react in some fashion to produce a solidparticle, such as a quantum dot. The reactants may also include, asfurther non-limiting examples, reactive chemicals, proteins,enzymes/substrates, nucleic acids, proteins/nucleic acids,enzymes/nucleic acids, acids/bases, antibodies/antigens,ligands/receptors, chemicals/catalysts, etc, as well as combinations ofthese and other reactants. As another example, one or both droplets maybe or contain one or more cells; for instance, if both droplets are (orcontain) cells, the two cells may be fused together, for instance, tocreate a hybridoma cell. As yet another example, one droplet that is orcontains a cell may be fused with another droplet to create a cellencapsulated in a fluid. Additionally, the fluid may be solidified insome cases to create a cell encapsulated in a solid. As still anotherexample, one droplet may be (or contain) a cell and the other dropletmay contain an agent to be delivered to the cell, such as a chemical, abiological molecule, a biological entity, etc., for instance, by fusinga droplet containing the agent with the cell. Non-limiting examplesinclude a nucleic acid (e.g., DNA or RNA, for example, for genetherapy), a protein, a hormone, a virus, a vitamin, an antioxidant, etc.

One embodiment of the present invention provides systems and methods fordetermining such reactions. As an example, a reaction may be determinedby measuring the distance and/or the time between where the reactantsare brought into contact and the time the reaction has reached a certainpoint. For example, with reference to FIG. 7C, two droplets 73, 74 mayeach contain reactants for a particular reaction. The two droplets arecoalesced together into a coalesced droplet containing fluidic regions71 and 72; thus, the reaction is initiated at a certain time and/or at acertain point within channel 70. As the droplet moves through channel70, the reactants may react. At a particular point d in the channel, thereaction may be determined to have been completed, e.g., by determininga color change within droplet 78. The reaction can then be determined asa function of time, and/or as a function of the distance traveled in thechannel (e.g., if the droplets move through channel at a constant and/ordeterminable or predictable rate). The term “determining,” as usedherein, generally refers to the analysis or measurement of a species,for example, quantitatively or qualitatively, or the detection of thepresence or absence of the species. “Determining” may also refer to theanalysis or measurement of an interaction between two or more species,for example, quantitatively or qualitatively, or by detecting thepresence or absence of the interaction. Example determination techniquesinclude, but are not limited to, spectroscopy such as infrared,absorption, fluorescence, UV/visible, FTIR (“Fourier Transform InfraredSpectroscopy”), or Raman; gravimetric techniques; ellipsometry;piezoelectric measurements; immunoassays; electrochemical measurements;optical measurements such as optical density measurements; circulardichroism; light scattering measurements such as quasielectric lightscattering; polarimetry; refractometry; turbidity measurements; etc.

In one embodiment, a reaction involving a fluidic droplet may be haltedand/or prevented from occurring or being completed. For example,counter-revolutionary flow within the droplet may prevent the reactionfrom occurring or being completed (e.g., due to separation of fluidsinvolved in the reaction), and/or the droplet may be “fixed” orimmobilized (for example, through solidification of at least a portionof the droplet) before the reaction can occur or be completed. As aparticular example, a fluidic droplet containing at least two reactants,each contained within a fluidic region of the fluidic droplet, may bedried in some fashion before substantial mixing of the fluids occurs(e.g., through spray-drying, through exposure to increased temperatures,etc.), thus creating a solidified particle containing separate regionstherein such that the reactants within each region are not able to reactwith each other. Thus, one embodiment of the invention provides a stableparticle comprising two or more separated components or regions that arereactive with each other, for example, as in a spray-dried particle.

In one set of embodiments, additional reactions or other steps may beperformed on a droplet (or other fluidic stream), for instance, beforeand/or after mixing of two or more droplets to form a coalesced droplet.For instance, in some cases, a fluidic droplet (or a portion thereof,such as a reactant) is reacted in some fashion (for example, with thesurrounding carrier fluid and/or an agent contained therein, with lightas in a photochemical reaction, etc.) before the fluidic droplet iscoalesced with other fluidic droplets. In another example, a fluidicregion of a coalesced droplet is reacted in some fashion, for example,with other fluidic region of the coalesced droplet and/or agentscontained therein, etc.

Combinations of various techniques, including those described above, mayalso be used to fabricate more complicated compositions. As an example,in FIG. 8, fluids 81, 82, 83, 84, and 85 are directed in an embodimentof the invention to form a fluidic droplet containing fluid region 86comprising fluids 81 and 82, and fluid region 87 comprising fluids 83,84, and 85. The fluidic droplet may further be hardened into a particlein some cases, e.g. as previously described.

Another aspect of the invention relates to discontinuous fluidicstreams, such as droplets, that are synchronously produced, i.e., wheretwo or more droplets are produced by two or more fluidic sources (e.g.,channel outlets), such that the droplets are each created at the sametime. In some cases, the droplets can be fused or coalesced, asdescribed herein. As examples, mechanical techniques may be used tosynchronize droplet production, an electric field may be modulated insome fashion to cause synchronous droplet formation, and/or theelectrical interaction of the fluids may be used to cause dropletformation from the two channels to be synchronous. One specific exampleof such a system is illustrated in FIG. 5B, where two water sources 57each synchronously produce droplets of water 55, 56 in carrier fluid 54within channels 53.

In one set of embodiments, synchronous droplet production may beachieved through mechanical techniques. For instance, the droplets maybe urged to detached from the fluidic sources (e.g., channel outlets)through mechanical forces, shaking, vibration (periodically, oraperiodically in some cases), or similar techniques known to those ofordinary skill in the art. The fluidic source and/or the channels thatthe droplets enter may be mechanically disturbed or agitated in somefashion to synchronize droplet production.

In another set of embodiments, the invention includes a system forelectrically synchronizing the release of two or more droplets from twoor more channels. For instance, an electric field that is used toinfluence droplet production (for instance, to produce droplets that arehighly charged or essentially saturated in charge, e.g., as in a Taylorcone) may be modulated in some fashion (“dithered”) to synchronizedroplet production. In one embodiment, the electric field may bemodulated with an AC field, for instance, an AC field (which may be anoffset AC field in some cases, i.e., the average of the AC field isnon-zero). Non-limiting examples of modulating AC fields are illustratedin FIGS. 9A-9C. In FIG. 9A, a square wave pulse is used to modulate theapplied electric field, while in FIG. 9B, a sine wave offset is used tomodulate the applied electric field, and in FIG. 9C, a sawtooth wave isused to modulate the applied electric field. Other suitable electricalmodulations (which may be periodic or aperiodic) can be readilyidentified by those of ordinary skill in the art. The modulations in theelectric field may cause the forming droplets to experience increasedelectrical forces, which may thus cause the droplets to synchronouslydetach from the channel.

In yet another set of embodiments, two (or more) channels are positionedsuch that an electrical interaction occurs between the droplets formingon the two channels. An example of such embodiments is schematicallyillustrated in FIGS. 9D-9G. In FIG. 9D, droplet 91 and droplet 92, eachforming from respective channels 93 and 94, are positioned, separated bya distance 95, such that an electrical interaction is able to occurbetween them, i.e., the droplets are positioned such that an electricalcharacteristic of one droplet, such as the charge, affects physicaland/or electrical characteristics of the other droplet, for example, theshape of the other droplet. In this embodiment, droplet 91 and droplet92 each have opposing electrical charges, such that the dropletsexperience an attractive force towards each other. As the droplets growin size (FIGS. 9E and 9F), the distance 95 between the dropletsdecreases, resulting in increased electrical interaction between thedroplets as the distance shortens. At some point (illustrated in FIG.9G), the interaction between the two forming droplets is such that theattractive force causes the droplets to detach from the channels.

In certain cases, the two droplets may then be allowed to fuse orcoalesce, as is schematically illustrated in FIG. 9H, where droplet 91and droplet 92 have been fused to form droplet 97. One specific exampleis illustrated in FIG. 5D, where two water sources 57 each synchronouslyproduce oppositely charged droplets of water 55, 56 within channels 53that fuse, forming droplets 57.

Another aspect of the invention generally relates to articles that canbe produced using any of the systems or methods described herein. Forinstance, in one set of embodiments, a fluidic stream (e.g., a dropletor a tube) may include aligned molecules, i.e., molecules substantiallyalignable by an electric field, for example, as is shown in FIG. 10 withmolecules 101 in a droplet (FIG. 10A) or a tube (FIG. 10B). Non-limitingexamples of alignable molecules include charged molecules, moleculeshaving a dipole moment, etc. The molecules may be aligned due to thepresence of the electric field, i.e., in the absence of an electricfield, the molecules would assume a random configuration or orientation.In some embodiments, the molecules are aligned due to the charge of thefluidic stream; i.e., the fluidic stream, when formed, may be highlycharged or essentially saturated in charge. The charge can cause themolecules to become aligned, e.g., during formation of the fluidicstream. In certain instances, the tube or droplet formed maysubsequently be hardened or otherwise immobilized in some fashion (e.g.,using a chemical reaction, polymerization, drying, etc.) such that themolecules remain substantially aligned.

In another set of embodiments, the invention generally relates to anemulsion. The emulsion may include droplets, such as those describedabove, and/or colloid particles. As used herein, an “emulsion” is givenits ordinary meaning as used in the art, i.e., a liquid dispersion. Insome cases, the emulsion may be a “microemulsion” or a “nanoemulsion,”i.e., an emulsion having a dispersant on the order of microns ornanometers, respectively. The dispersion or emulsion, in some cases, mayinclude droplets having a homogenous distribution of diameters, i.e.,the droplets may have a distribution of diameters such that no more thanabout 10%, about 5%, about 3%, about 1%, about 0.03%, or about 0.01% ofthe droplets have an average diameter greater than about 10%, about 5%,about 3%, about 1%, about 0.03%, or about 0.01% of the average diameterof the droplets. As one example, such an emulsion may be created byallowing fluidic droplets of the appropriate size or sizes (e.g.,created as described herein) to enter into a solution that is immisciblewith the fluidic droplets.

The formation of the emulsion can be controlled by those of ordinaryskill in the art, based on the teachings herein, as well as availableteachings in the field known to those of ordinary skill in the art, forexample, flow-focusing. Reference can be made, for example, toGanan-Calvo, et al., “Generation of Steady Liquid Microthreads andMicron-Sized Monodispersed Sprays and Gas Streams,” Phys. Rev. Lett.,80:2 (Jan. 12, 1998), as well as numerous other texts, for the selectionof fluids to carry out the purposes of the invention. Control of theentering fluid flow rate, and/or the ratio between the flow rates ofdispersing and subject fluids, can be used to control the subjectfluidic stream and/or droplet size, and monodispersity versuspolydispersity in the final emulsion, in some cases.

The invention, in yet another set of embodiments, generally relates tofluidic streams containing lipids, for instance, liposomes, lipid tubes,etc. In one embodiment, a lipid fluidic stream, such as a tube or adroplet, can be produced in an aqueous or hydrophobic carrier fluid,e.g., using any of the systems and methods described herein. In anotherembodiment, a lipid tube may be produced by stretching a liposome or alipid droplet.

In some embodiments, the invention provides systems and methods forproducing colloidosomes, for example, as described in InternationalPatent Application No. PCT/US01/46181, filed May 25, 2001, entitled“Methods and Compositions for Encapsulating Active Agents,” by Weitz, etal., published as Publication No. WO 02/47665 on Jun. 20, 2002,incorporated herein by reference. In one embodiment, a charged fluidicdroplet is directed at a solution comprising colloids, which colloidsmay be oppositely charged. Charge interaction of the droplet with thecolloids may cause the colloids to collect on the surface of thedroplet, thereby forming a colloidosome in solution. An example of thisprocess can be seen in FIG. 10C, where an electrically charged fluidicdroplet 105 is directed at a solution 106 containing colloids 107.Colloids 107 may be oppositely charged relative to fluidic droplet 105.The electrical attraction between fluidic droplet 105 and colloids 107may cause the colloids 107 to be attracted to the surface of fluidicdroplet 105, thereby forming colloidosome 108. In some cases, thefluidic droplet may also be hardened into particle, e.g. as previouslydescribed.

Any of the above-described embodiments can be used in a variety ofapplications, for example, medicine (e.g., pharmaceuticals), skin careproducts (e.g. lotions, shower gels), foods (e.g. salad dressings, icecream), ink encapsulation, micro-templating of micro-engineeredmaterials (e.g., photonic crystals, smart materials, etc.), and thelike. Additionally, in some cases, any of the above-described fluidicstreams (e.g., droplets or tubes) may include cells or therapeuticagents. In some applications, highly monodisperse and/or concentratedliquid crystal and/or precipitated droplets may also self-organize intotwo and three dimensional structures, for example, for use in noveloptical devices.

A variety of materials and methods can be used to form certaincomponents of the invention, according to yet another aspect. In somecases various materials selected lend themselves to various methods. Forexample, components of the invention can be formed from solid materials,in which the channels can be formed via micromachining, film depositionprocesses such as spin coating and chemical vapor deposition, laserfabrication, photolithographic techniques, etching methods including wetchemical or plasma processes, and the like. See, for example, Angell, etal., Scientific American, 248:44-55 (1983). In one embodiment, at leasta portion of the system is formed of silicon by etching features in asilicon chip. Technology for precise and efficient fabrication ofdevices of the invention from silicon is known. In another embodimentthat section (or other sections) can be formed of a polymer, and can bean elastomeric polymer, or polytetrafluoroethylene (PTFE; Teflon®), orthe like.

Different components can be fabricated of different materials. Forexample, a base portion including a bottom wall and side walls can befabricated from an opaque material such as silicon orpolydimethylsiloxane (PDMS), and a top portion can be fabricated from atransparent material such as glass or a transparent polymer, forobservation and control of the fluidic process. Some components can becoated so as to expose a desired chemical functionality to fluids thatcontact interior channel walls, where base supporting material does nothave the precise, desired functionality. For example, components can befabricated as illustrated, with interior channel walls coated withanother material.

The material used to fabricate devices of the invention, or materialused to coat interior walls of fluid channels, may desirably be selectedfrom among those materials that will not adversely affect or be affectedby fluid flowing through the device, e.g., material(s) that ischemically inert in the presence of fluids to be used within the device.

In one embodiment, components of the invention are fabricated frompolymeric and/or flexible and/or elastomeric materials, and can beconveniently formed of a hardenable fluid, facilitating fabrication viamolding (e.g. replica molding, injection molding, cast molding, etc.).The hardenable fluid can be essentially any fluid that can be induced tosolidify, or that spontaneously solidifies, into a solid capable ofcontaining and transporting fluids. In one embodiment, the hardenablefluid comprises a polymeric liquid or a liquid polymeric precursor (i.e.a “prepolymer”). Suitable polymeric liquids can include, for example,thermoplastic polymers, thermoset polymers, or mixture of such polymersheated above their melting point; or a solution of one or more polymersin a suitable solvent, which solution forms a solid polymeric materialupon removal of the solvent, for example, by evaporation. Such polymericmaterials, which can be solidified from, for example, a melt state or bysolvent evaporation, are well known to those of ordinary skill in theart. A variety of polymeric materials, many of which are elastomeric,are suitable, and are also suitable for forming molds or mold mastersfor producing components of the invention, for embodiments where one orboth of the mold masters is composed of an elastomeric material. Anon-limiting list of examples of such polymers includes polymers of thegeneral classes of silicone polymers, epoxy polymers, and acrylatepolymers. Epoxy polymers are characterized by the presence of athree-membered cyclic ether group commonly referred to as an epoxygroup, 1,2-epoxide, or oxirane. For example, diglycidyl ethers ofbisphenol A can be used, in addition to compounds based on aromaticamine, triazine, and cycloaliphatic backbones. Another example includesthe well-known Novolac polymers. Examples of silicone elastomerssuitable for use according to the invention include those formed fromprecursors including the chlorosilanes such as methylchlorosilanes,ethylchlorosilanes, and phenylchlorosilanes, and the like.

Silicone polymers are preferred in one set of embodiments, for example,the silicone elastomer polydimethylsiloxane (PDMS). Exemplarypolydimethylsiloxane polymers include those sold under the trademarkSylgard by Dow Chemical Co., Midland, Mich., and particularly Sylgard182, Sylgard 184, and Sylgard 186. Silicone polymers including PDMS haveseveral beneficial properties simplifying fabrication of themicrofluidic structures of the invention. First, such materials areinexpensive, readily available, and can be solidified from aprepolymeric liquid via curing with heat. For example, PDMSs aretypically curable by exposure of the prepolymeric liquid to temperaturesof about, for example, 65° C. to about 75° C. for exposure times ofabout, for example, 1 hour. Second, silicone polymers, such as PDMS, areelastomeric and are thus useful for forming very small features withrelatively high aspect ratios, necessary in certain embodiments of theinvention. Flexible (e.g. elastomeric) molds or masters can beadvantageous in this regard.

Another advantage of forming structures such as microfluidic structuresof the invention from silicone polymers, such as PDMS, is the ability ofsuch polymers to be oxidized, for example by exposure to anoxygen-containing plasma such as an air plasma, so that the oxidizedstructures contain at their surface chemical groups capable ofcross-linking to other oxidized silicone polymer surfaces or to theoxidized surfaces of a variety of other polymeric and non-polymericmaterials. Thus, components can be fabricated and then oxidized andessentially irreversibly sealed to other silicone polymer surfaces, orto the surfaces of other substrates reactive with the oxidized siliconepolymer surfaces, without the need for separate adhesives or othersealing means. In most cases, sealing can be completed simply bycontacting an oxidized silicone surface to another surface without theneed to apply auxiliary pressure to form the seal. That is, thepre-oxidized silicone surface acts as a contact adhesive againstsuitable mating surfaces. Specifically, in addition to beingirreversibly sealable to itself, oxidized silicone such as oxidized PDMScan also be sealed irreversibly to a range of oxidized materials otherthan itself including, for example, glass, silicon, silicon oxide,quartz, silicon nitride, polyethylene, polystyrene, glassy carbon, andepoxy polymers, which have been oxidized in a similar fashion to thePDMS surface (for example, via exposure to an oxygen-containing plasma).Oxidation and sealing methods useful in the context of the presentinvention, as well as overall molding techniques, are described inDuffy, et al., “Rapid Prototyping of Microfluidic Systems andPolydimethylsiloxane,” Analytical Chemistry, 70:474-480 (1998),incorporated herein by reference.

Another advantage to forming microfluidic structures of the invention(or interior, fluid-contacting surfaces) from oxidized silicone polymersis that these surfaces can be much more hydrophilic than the surfaces oftypical elastomeric polymers (where a hydrophilic interior surface isdesired). Such hydrophilic channel surfaces can thus be more easilyfilled and wetted with aqueous solutions than can structures comprisedof typical, unoxidized elastomeric polymers or other hydrophobicmaterials.

In one embodiment, a bottom wall is formed of a material different fromone or more side walls or a top wall, or other components. For example,the interior surface of a bottom wall can comprise the surface of asilicon wafer or microchip, or other substrate. Other components can, asdescribed above, be sealed to such alternative substrates. Where it isdesired to seal a component comprising a silicone polymer (e.g. PDMS) toa substrate (bottom wall) of different material, it is preferred thatthe substrate be selected from the group of materials to which oxidizedsilicone polymer is able to irreversibly seal (e.g., glass, silicon,silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, epoxypolymers, and glassy carbon surfaces which have been oxidized).Alternatively, other sealing techniques can be used, as would beapparent to those of ordinary skill in the art, including, but notlimited to, the use of separate adhesives, thermal bonding, solventbonding, ultrasonic welding, etc.

The following example is intended to illustrate certain aspects ofcertain embodiments of the present invention, but does not exemplify thefull scope of the invention.

EXAMPLE

In this example, a device of the invention was used for the applicationof an electric field at a junction between two immiscible fluids. Theelectric field created charged droplets and large forces necessary foremulsification, while the junction stabilized droplet production even athigh fields, when a Taylor cone was present. Applications of thistechnology include, but are not limited to, the generation of chargeddroplets with a narrow distribution in radius down to submicron sizesand controlled droplet coalescence by oppositely charged droplets.

The device of this example was created by patterning PDMS 113 on a glasssubstrate 112 having electrodes 111 formed from indium tin oxide (“ITO”)(FIG. 11A), forming the structure schematically shown in FIG. 11B. Avoltage difference 115 was applied to the electrodes (FIG. 11B) tocreate an applied electric field (FIG. 11C). The device included atwo-fluid injection system where a conductive fluid was injected into anon-conductive fluid in the presence of the electric field to generatedroplets of the conductive fluid dispersed in the non-conductive fluid.Droplets could be created having diameters of less than about 1 micronto about 100 microns. These droplets remain charged with the sign of thecharge dependent on the sign of the electric field with respect to thedirection of flow, as is shown in FIG. 11C.

Images of droplets generated by this device are shown in FIGS. 11D-11I.In the absence of an electric field, as illustrated in FIGS. 11D-11G,large droplets were generated, while in the presence an electric field(E=2 V/micron), a Taylor cone was stabilized with uniform submicrondroplets being emitted from the tip, as shown in FIGS. 11H and 11I. Thedroplets may also be discharged on a ground electrode located furtherdownstream. Such a device has many applications, for example, ingenerating well controlled nanoemulsions.

Oppositely oriented devices were also used to generate droplets havingopposite sign of charge. Using this charge, the droplets coalesced at aprecise or generally predetermined location, as is shown in FIGS. 5B and5D. In FIG. 5B, there is no electric field applied and the droplets didnot coalesce. In FIG. 5E-5G, the electrostatic attraction can cause thedrops to coalesce. The electric field, in some cases, can be used tocontrol the phase between when the droplets are generated to ensuresimultaneous arrival at a central location and subsequent coalescence,for example, through an auto feedback mechanism or a using an AC dither.As shown in FIGS. 6A-6D, the surface of the droplets could be deformedand electrostatic forces may overcome surface tension to produce a fluidbridge to coalesce and/or neutralize the droplets.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

It should also be understood that, unless clearly indicated to thecontrary, in any methods claimed herein that include more than one act,the order of the acts of the method is not necessarily limited to theorder in which the acts of the method are recited.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method of producing a plurality of droplets,comprising: flowing a first liquid in a first microfluidic channel and asecond liquid in a second microfluidic channel such that the first andthe second liquids meet at a junction of the first and secondmicrofluidic channels; and producing a plurality of regularly-spaced,single-file droplets comprising the first liquid at the junction byapplying an electric field to the first and second liquids as they meetat the junction, wherein the droplets are surrounded by the secondliquid and flow within at least one of the microfluidic channels.
 2. Themethod of claim 1, wherein said first liquid is an aqueous liquid andsaid second liquid is a liquid that is immiscible in said aqueousliquid.
 3. The method of claim 1, wherein said droplets have an averagediameter of less than about 60 microns.
 4. The method of claim 1,wherein said first microfluidic channel and said second microfluidicchannel are contained on a device comprising a location for expellingsaid droplets.
 5. The method of claim 1, further comprising the step ofconducting a reaction in said droplets.
 6. The method of claim 1,wherein said first liquid forms a Taylor cone when expelled from anoutlet in said first microfluidic channel.
 7. The method of claim 6,wherein the first and second liquids flow through a dimensionalrestriction that stabilizes the Taylor cone.
 8. The method of claim 1,wherein the droplets comprise a homogeneous distribution of diameters.